1. Field of the Invention
The present invention relates to an optical information reproducing apparatus. More specifically, the present invention relates to an improvement in an optical servo system applicable to a radial tracking apparatus, a focusing apparatus or the like in a video disk player or the like.
2. Description of the Prior Art
A radial tracking apparatus, a focusing apparatus and the like have been proposed and put into practice for use with a video disk player or the like. An example of a radial tracking apparatus for use with a video disk player will be described in the following.
A conventional radial tracking apparatus is basically structured such that a reading laser beam is divided into three beams by means of a diffraction grating, three light spots are prepared by using one objective, the central spot being projected to the center of a track being traced on a disk for the purpose of reading and the two spots at both sides being projected such that the track is sandwiched from both sides, and the reflected light of these three light spots is condensed by the same objective, whereby the three reflected light beams are focused on three photodetectors, respectively, and an information signal is obtained from the central photodetector and an error output of radial tracking is obtained by evaluating the difference output of the photodetectors at both sides. If and when the track is positioned at the center of the read light spot, the light beams impinging on the photodetectors at both sides have the same intensity and accordingly the error output becomes zero. However, if and when the track position drifts left or right of the center of the read light spot, the intensities of the light beams incident on both photodetectors become unbalanced, whereby an error output is obtained. Therefore, the position of the light spot is adjusted so that the error output may be zero.
The above described conventional apparatus, however, involves shortcomings that require delicate optical components such as a diffraction grating and three photodetectors and further necessitates fine adjustment of a rotational angle of the diffraction grating and a three dimensional position of each of the three photodetectors.
An improvement for more or less eliminating such shortcomings has also been proposed, in an apparatus which is referred to as a wobbling type radial tracking apparatus. An example of the so-called wobbling type radial tracking apparatus is shown in FIG. 1. Referring to FIG. 1, the radial tracking apparatus shown comprises a laser beam source 1, a wobbling mirror 2 provided to be turnably vibrated in the direction of an arrow 15 by means of a piezoelectric element 13, a beam-splitter 3, a tracking mirror 4 provided to be turnably driven in the direction of an arrow 16 by means of an actuator 5, an objective 6 for forming a light spot 7, a disk 8 including tracks 9 of a series of pits recorded on the disk 8, a photodetector 10 for receiving a light beam reflected from the track 9, a phase detector 11, an oscillator 12 for providing an oscillator signal of the frequency f to a piezoelectric element 13, and a servo amplifier 14.
In operation, when a voltage of the frequency f obtained from the oscillator 12 is applied to the piezoelectric element 13 so that the wobbling mirror 2 is vibrated, the light beam from the light source 1 undergoes minor deflection at the frequency f. Therefore, the light spot 7 formed by the objective 6 is subjected to a minor amount of vibration at the frequency f in the direction transversing the track 9, which is referred to as wobbling. The light beam reflected from the track 9 at that time is separated from the incident light beam by means of the beam-splitter 3 and is detected by the light detector 10. As a result, an output characteristic as shown as (b) in FIG. 2 is obtained. The direct current intensity E.sub.D of the reflected light beam when the light spot 7 crosses the pit 9a of the track in the direction of arrow X as seen at (a) in FIG. 2 is as shown at (b) in FIG. 2. Accordingly, the alternating current intensities E.sub.WL and E.sub.WR obtained when the light spot 7 is subjected to minor vibration S are as shown at (b) in FIG. 2. It is seen that the waveform is reversed by 180.degree. depending on whether the light spot 7 has deviated leftward or rightward from the center of the track. Therefore, referring to FIG. 1, when the output of the photodetector 10 is applied to the phase detector 11 for the purpose of phase detection using the output from the oscillator 12 as a reference signal, a track error signal E.sub.W as shown at (c) in FIG. 2 is obtained at the output of the phase detector 11. The track error signal is amplified by the servo amplifier 14 and the amplified output is applied to the actuator 5 for driving the tracking mirror 4 by properly selecting the polarity, so that the X directional position of the light spot 7 is controlled so as to be always positioned on the track 9.
The above described conventional apparatus has been simplified as compared with the fundamental type; however, such a wobbling tracking apparatus suffers from a number of shortcomings as set forth in the following. More specifically, it is required to wobble the light beam at the high speed, say normally 20 to 50 kHz, and it is extremely difficult to do so. Although a combination of a piezoelectric element and a mirror, as exemplified, is an effective approach, as a matter of practice numerous problems are involved that an adhering process is required for combining the mirror with the element, the vibration phase varies due to the temperature, optical adjustment is difficult and the like. In addition, it is unavoidable that the wobbling causes interference in a reproduced signal and hence a further disadvantage is involved that a beat interference is caused in the case of a video signal.